Machine tools, in particular electric tools, such as cordless screwdrivers, cordless combi drills, and cordless impact drills, have a drive motor, usually an electric motor, which drives a spindle via a single-stage or multi-stage gear unit. The spindle contains a tool holder. It is also possible for the tool holder to be fastened to the spindle in a rotationally fixed, but detachable, manner. Frequently the machine tools, in particular hand-held electric tools, have a spindle lock driver. The spindle lock driver is situated in the drive train between the drive motor and the spindle and allows the spindle to rotate in the housing if the torque applied to the spindle from the outside is less than from a driver on the gear unit side. If the torque applied from the tool holder to the spindle from the outside is greater than from the driver on the gear unit side, the spindle lock driver locks the spindle with respect to the housing. This allows, for example, a drill chuck to be opened without additional manual operation of the spindle lock, or the performance of work cases requiring higher torque than the electric motor is able to generate. Furthermore, it is possible to remove drill bits from a bore by rotating the machine tool.
A machine tool having an electric motor, which drives a spindle having a chuck via a countershaft gear unit, is known from German Patent Publication DE 699 21 250 T2. The countershaft gear unit is made up of a pinion which is connected in a rotationally fixed manner to the motor shaft and meshes with a countershaft gear wheel which is rotationally supported on a countershaft. Another countershaft pinion, which fixedly rests on the countershaft, meshes with an output gear wheel which is rotationally fixed to the spindle.
A spindle lock driver which includes a prismatic output part is accommodated in a cylindrical chamber fixed to the housing which includes the countershaft on the input side. Longitudinal grooves which engage the axially directed finger-like drivers having play in the circumferential direction are provided on the edges of the prism. The drivers are formed on the countershaft gear wheel. Clamping rollers are provided between the prism surfaces and the inner wall of the cylindrical chamber or a bushing which is fixed to the housing and situated in the chamber. In normal operation, the torque of the electric motor is transferred via the pinion to the countershaft wheel and from there via the driver to the output part which rests fixedly on the countershaft. The countershaft pinion is driven by the countershaft, the countershaft pinion meshing with the output gear wheel and thus driving the spindle. If the torque transferred from the output gear wheel to the countershaft is greater than the torque transferred from the countershaft wheel, the output part twists against the drivers within the predefined play and the clamping rollers exert a clamping action between the output part and the cylindrical chamber fixed to the housing, so that the torque generated by the spindle is supported on the housing and blocks the spindle.